This invention relates to permanent magnet, direct current motors which are rendered self-starting.
It is possible in direct current motors with permanent magnets, ordinarily on the rotor, after de-energization of the motor, for the rotor to come to rest in a position where there will be no torque on the rotor upon re-energization of the motor. Such a position is termed a cogged position. Positions where there is no torque on the rotor upon re-energization of the motor are the positions where the axis of the magnetic field produced by the rotor is either in the same direction as or in the opposite direction from the axis of the magnetic field produced by the stator. In other words, there is no torque on the rotor upon re-energization of the motor if the axis of the magnetic field produced by the rotor is either completely in phase with (0 electrical degrees) or completely out of phase with (180 electrical degrees) the axis of the magnetic field produced by the stator. In either case, the motor will not start. Moreover, saliencies of the wound motor component are likely to cause the rotor to come to rest at just the cogged position, this being the position of least reluctance, unless the motor design has taken cogging into account.
One technique that is used to make a permanent magnet, direct current motor self-starting is to provide permanent magnets on the stator to attract the permanently magnetized rotor magnets, so that after motor de-energization the rotor is angularly offset from a cogged position. However, use of this technique results in a motor that does not run smoothly.
Another technique that is used to make a permanent magnet, direct current motor self-starting is to provide reluctance modifications in the magnetic circuit to position the rotor after motor de-energization so that the rotor is angularly offset from a cogged position. Suggested reluctance modifications used to position the rotor have included manual means, use of auxiliary windings, use of stator pole pieces that have a varying reluctance in the direction of rotor movement, and use of a rotor and a stator wherein the width of the air gap between the rotor and the stator is nonuniform.
However, there are problems with using these reluctance modifications. Use of manual means results in a cumbersome, complex motor that requires an operator. Use of auxiliary windings results in a complex motor. Use of pole pieces that have a varying reluctance and use of a rotor and a stator where the width of the air gap between the rotor and the stator is nonuniform may necessitate the use of more costly manufacturing processes or may result in very little displacement from the cogged position.